In conventional information recording media, the information layer (phase-change material layer) is a phase-change information recording medium that utilizes the phenomenon in which a phase-change between a crystalline phase and an amorphous phase is produced. Among these phase-change information recording media, one in which information is recorded, erased, overwritten and/or reproduced optically by using a laser beam is an optical phase-change information recording medium. In an optical phase-change information recording medium, a change of state between a crystalline phase and an amorphous phase in the phase-change material of the recording layer is produced as a result of heat generated by irradiation with a laser beam, and the difference in reflectance between the crystalline phase and amorphous phase is detected and is read out as information. Among optical phase-change information recording media, in a overwritable optical phase-change information recording medium in which information can be erased and overwritten, the initial state of the recording layer is generally a crystalline phase, and when information is recorded, the recording layer that is irradiated with a high power (recording power) laser beam melts and then rapidly cools, so that the laser-irradiated portion becomes amorphous. On the contrary, when information is erased, the recording layer that is irradiated with a laser beam with a power that is lower than that during recording (erasing power) is warmed and cooled, so that the laser-irradiated portion becomes crystalline. Consequently, in a rewritable optical phase-change information recording medium, by irradiating the recording layer with a laser beam for which the power can be modulated between a high power and a low power, it is possible for new information to be recorded or overwritten while recorded information is being erased. Moreover, among optical phase-change information recording media, for write-once optical phase-change information recording media in which it is possible for information to be recorded one time but not possible for information to be erased or overwritten, the initial state of the recording layer is generally an amorphous phase, and the laser-irradiated portion becomes crystalline as the recording layer warms up and cools while being irradiated with a high power (recording power) laser beam when information is recorded.
Instead of the abovementioned irradiation with a laser beam, there are also electrical phase-change information recording media that record information by causing a state change in the phase-change material of the recording layer by means of Joule heating generated by the application of electrical energy (for example electrical current). In these electrical phase-change information recording media, the phase-change material of the recording layer undergoes a state change between a crystalline phase (low resistance) and an amorphous phase (high resistance) by means of Joule heating generated by the application of electrical current, and the difference in electrical resistance between the crystalline phase and amorphous phase is detected and is reproduced as information.
The commercial 4.7 GB/DVD-RAM is given by the present inventors as an example of an optical phase-change information recording medium. As shown in FIG. 12 for information recording medium 12, the GB/DVD-RAM has a 7-layer configuration, where first dielectric layer 2, first interface layer 3, recording layer 4, second interface layer 5, second dielectric layer 6, light absorption correction layer 7, and reflection layer 8 are provided over substrate 1 in order from the laser incident side.
First dielectric layer 2 and second dielectric layer 6 adjust the optical path and enhance the light absorption efficiency of recording layer 4, so that optical action increases the magnitude of the signal strength as the change in reflectance between the crystalline phase and the amorphous phase grows larger, and serve a thermal function to insulate the heat-sensitive substrate 1 and dummy substrate 10 and so forth from the heat due to the higher temperature of recording layer 4 during recording. In use now more than previously, (ZnS)80(SiO2)20 (mol %) is a superior dielectric material that has transparency and a high refractive index, and is also a good insulator with low thermal conductivity, favorable mechanical characteristics and resistance to humidity. Furthermore, the layer thicknesses of first dielectric layer 2 and second dielectric layer 6 can be determined exactly according to a calculation based on the matrix method, so as to satisfy conditions that increase the change in the amount of reflected light between the crystalline phase and amorphous phase of recording layer 4, and increase the light absorption in recording layer 4.
By using a high crystallization speed material in recording layer 4 that includes (Ge—Sn)Te—Sb2Te3 wherein Sn substitutes for a portion of the Ge in the pseudo-binary phase-change material GeTe—Sb2Te3 that combines the compounds GeTe and Sb2Te3, not only is there efficient overwriting of the initial recording, but superior recording shelf life (the indicator of whether the recorded signal can be recovered after long-term storage) and overwriting shelf-life (the indicator of whether the recorded signal can be erased or overwritten after long-term storage) are also realized.
First interface layer 3 and second interface layer 5 function to prevent mass transfer from taking place between first dielectric layer 2 and recording layer 4, and between second dielectric layer 6 and recording layer 4. In this mass transfer phenomenon, when (ZnS)80(SiO2)20 (mol %) is used in first dielectric layer 2 and second dielectric layer 6, S (sulfur) diffuses into the recording layer during the time when recording layer 4 is irradiated with a laser beam for repeated recording and overwriting. When S diffuses into the recording layer, the repeat overwriting capability deteriorates. The use of Ge-containing nitrides in first interface layer 3 and second interface layer 5 favors the avoidance of this deterioration of the repeat overwriting capability (for example, see Patent Document 1).
Through the use of technology such as that described above, superior overwriting performance and high reliability were achieved and the 4.7 GB/D VD-RAM was brought to commercialization.
Moreover, various kinds of technology have been studied in order to obtain information recording media with higher capacity. In the example of optical phase-change information recording media, a high density recording technique with a smaller laser beam spot diameter was investigated by using a violet-blue laser with a shorter wavelength than that of the conventional red laser, and by using a thinner substrate on the laser beam-incident side and an objective lens with a larger numerical aperture (NA). When recording is carried out with a smaller spot diameter, since the laser beam irradiation can be limited to a smaller region, the volume change will be greater with an increased power density being absorbed by the recording layer. Consequently, it becomes easier for mass transfer to occur, and when S-containing materials such as ZnS—SiO2 are used in the vicinity of the recording layer, the repeat overwriting capability will deteriorate.
In addition, the information capacity increases two-fold by using an optical phase-change information recording medium that is provided with two information layers (referred to below as a bilayer optical phase-change information recording medium), and the technique of carrying out record/reproduce operations on the two information layers by using an incident laser beam from one side has also been investigated (for example see Patent Documents 2 and 3). In these bilayer optical phase-change information recording media, laser beam that is used will pass through the information layer proximal to the laser beam incident side (referred to as the first information layer) in order to perform record/reproduce operations on the information layer distal to the laser beam incident side (referred to below as the second information layer), so the first information layer should have an extremely thin film thickness and high permeability. However, because the effect of mass transfer from a layer in the vicinity of the recording layer becomes more significant as that recording layer becomes thinner, there will be a marked deterioration in the repeat overwriting capability when using S-containing materials such as ZnS—SiO2 in the vicinity of the recording layer.
Heretofore, in cases such as that described above, inventors have introduced Ge-containing nitrides in interface layers on both sides of the information layer in substantially the same manner as with the 4.7 GB/DVD-RAM, so that the effect of mass transfer was mitigated and the deterioration of the repeat overwriting capability was avoided.
Nevertheless, when carrying out high density recording operations with a smaller laser beam spot diameter in optical phase-change information recording media, the recording layer is irradiated with a higher energy (laser power) when information is recorded. For this reason, when conventional Ge-containing nitrides are used in the interface layer, the heat generated in the recording layer gives rise to film disruption in that interface layer, and there is a problem with marked deterioration of the repeat overwriting capability due to the interface layer becoming unable to control the diffusion of S from the accompanying dielectric layer.
Moreover, since Ge-containing nitrides have high thermal conductivity, if a thicker interface layer is constructed in order to control the diffusion of S from the dielectric layer, the heat will facilitate the diffusion. As a result, there will be a problem with reduced recording sensitivity.
Patent Document 1: Japanese published unexamined patent application No. H10-275360 (pp. 2-6, FIG. 2) (1998)
Patent Document 2: Japanese published unexamined patent application No. 2000-36130 (pp. 2-11, FIG. 2)
Patent Document 3: Japanese published unexamined patent application No. 2002-144736 (pp. 2-14, FIG. 3)